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To maximise throughput of the spectrograph it is important
to keep the science object in the slit. Since SpeX is
optimised to use slit widths about the same size as the
seeing it is necessary to use active guiding rather than
rely on telescope tracking alone.
OBSERVERS SHOULD COME PREPARED WITH SUITABLE GUIDE STARS.
Guiding can be done on spill-over flux from the object in the slit or on an object in the 60x60 arcsec field of the imager. The magnitude limit for auto-guiding on a star in the slit is about J=15. The magnitude limit for manual guiding (using the paddle) on a star in the slit is about J=18. The magnitude limit for auto-guiding on a star in the field is about J=18. These limits depend on seeing and slit width. Auto-guiding also works very well on extended objects such as galaxy nuclei and small disks (eg. 4 arcsec diameter Uranus) so long as the objects are bright enough and guiding on the peak or centroid is acceptable. Positioning a slit at a particular location on Jupiter or Saturn, for example, must be done using non-sidereal telescope tracking with manual updates provided by observing the slit location in the SpeX slit-viewer. For details read the document 'How to set up the IR guider'.
Visible guiding can be done using the off-axis guider.
This guider has an annular field-of-view with an inner
radius of 100 arcsec and an outer radius of 200 arcsec.
Magnitude limit Itime
(no moon) (sec)
V=10.0 0.7
V=12.7 3.5
V=14.0 6.5
V=15.0 8.0
Place the science object in the slit and ask the telescope
operator to start guiding on a suitable off-axis star.
Make sure that the science object does not move out of the
slit when guiding is started (it should not). Set up the
telescope nod position as described in
'How to set up the the IR guider'.
Visible guiding can be done using the tip-tilt guider.
This guider has an 80 x 80 arcsec FOV. Due to the
fragility of the tip-tilt wavefront sensor package
tip-tilt guiding has seen little use and we urge
observers not to assume it will be working.
In the SpeX XUI move a dichroic into the beam. There is
a choice of two:
1) 0.80 micron cut-on which sends wavelengths < 0.80 micron
to the wavefront sensor (less light to guide on but
spectral coverage > 0.8 micron)
2) 0.95 micron cut-on which sends wavelengths < 0.95 micron
to the wavefront sensor (more light to guide on but
less spectral coverage > 0.95 micron)
Place the science object in the slit and ask the telescope
operator to start guiding on a suitable off-axis star.
Make sure that the science object does not move out of the
slit when guiding is started (it may need slight repositioning
using tip-tilt guide cursor buttons - ask TO). Set up the
telescope nod position as described in 'How to set up the
the IR guider'.
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